Pressure actuable switch apparatus with bellows and fluid damping means

ABSTRACT

Pressure actuable switch apparatus is disclosed as comprising a switch mechanism, an envelope for receiving the switch mechanism therein and having a portion displaceable in response to increased external pressure, and a translating mechanism responsive to the displacement of the envelope portion for moving an arm of the switch mechanism from a first position to a second position. First and second electrical contacts are mounted within the envelope to engage electrically the arm when it is disposed respectively at the first and second positions. Illustratively, the translating mechanism includes a spline housing defining at least one spline, an actuating cam member coupled with the envelope portion for movement therewith, and a ratchet cam for moving the arm. The actuating and ratchet cams each have keys and are disposed within the spline housing so that the respective keys are channeled within the spline thereof for controlled movement as the envelope portion is displaced, to move the actuating cam to engage the ratchet cam. The actuating and ratchet cams include engaging cam surfaces, illustratively in the form of sets of mutually-engaging teeth. The key of one of the actuating and ratchet cams is displaced from the position of the other key with respect to its cam surface, whereby the displacement of the envelope portion and the actuating cam as external pressure increases, directs the ratchet cam and arm from the first to the second position. The spline housing may be provided with a plurality of splines whereby the ratchet cam may be successively moved from the first to the second to any of a plurality of positions upon repeated raising and lowering of the external pressure.

United States Patent [191 Maxwell, Jr. et al.

George W. Gillepsie, West Mifflin, both of Pa.

[73] Assignee: Westinghouse Electric Corporation Pittsburgh, Pa.

[22] Filed: Mar. 6, 1973 [21] Appl. No.: 338,488

[52] US. Cl 200/83 C, 92/41, 200/34, 200/83 D, 200/83 T [51] Int. Cl.H01h 35/34 [58] Field of Search 200/156, 83 P, 83 C, 81 R, 200/153 J,34, 83 T, 83 D; 92/41; 102/70 B OTHER PUBLICATIONS Product Engineering,June 1951, ThirtySe ven Ideas for Application of Precision Snap-ActingSwitches; pp. 3 & 4. f

Primary Examiner-Robert K. Schaffer Assistant Ex-aminerGerald P. TolinAttorney, Agent, or FirmD. Schron [5 7] ABSTRACT Pressure actuableswitch apparatus is disclosed as comprising a switch mechanism, anenvelope for receiving the switch mechanism therein and having a portiondisplaceab le in response to increased external pressure, and atranslating mechanism responsive to the displacement of the envelopeportion for moving an arm of the switch mechanism, from a first positionto a second position. First and second electrical contacts are mountedwithin the envelope to engage electrically the arm when it is disposedrespectively at the first and second positions. Illustratively, thetranslating mechanism includes a spline housing defining at least onespline, an'actuating cam member coupled with the envelope portion formovement therewith, and a ratchet cam for moving the arm. The actuatingand ratchet cams each havekeys and are disposed within the splinehousing so that the respective keys are channeled within the splinethereof for controlled movement as the envelope portion is displaced, tomove the actuating cam to engage the ratchet cam. The actuating andratchet cams include engaging cam surfaces, illustratively in the formof sets of mutuallyengaging teeth. The key of one of the actuating andratchet cams is displaced from the position of the other key withrespect to its cam surface, whereby the displacement of the envelopeportion and the actuati ng cam as external pressure increases, directsthe ratchet'cam and arm from the first to the second position. Thespline housing may be provided with a plurality of splines whereby theratchet cam may be successivelyimoved fromthe first to the second to anyof a'plurality of positions upon repeated raising and lowering of theexternal pressure.

2 Claims, 16 Drawing Figures PATENIEL W 3 4 XMEET 20! 6 PATENIELEEE awnSHEEY 30? 6 Pmtmm r: 3.852546 sum we a FIG4G FIG. 4F

PATENIE' EEC 31974 METSOFS PRESSURE ACTUABLE SWITCH APPARATUS WITHBELLOWS AND FLUID DAMPING MEANS BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to switching apparatus and inparticular to such switching apparatus actuatable inresponse to a changeof external pressure.

2. Description of the Prior Art There is known a class of aeriallydelivered weapons such as mines or bombs having a container forreceiving a plurality of individual component weapons. The componentweapons are released from the dispenser during the delivery ordeployment. Typically, each component weapon has a separate fusingdevice associated therewith for controlling arming delay, detonationtime, etc. Such fusing devices are preset prior to their delivery. Atpresent, in order to set each of the plurality of the fusing devicesassociated with a great number of component weapons, it is necessary tomanually set a switch on each of the fusing devices, prior to itsloading into the dispenser. A change inthe settings of the fuse devicesrequires the unloading of the multiple component weapons and the manualmanipulation of the switches associated with each fuse device.

SUMMARY OF THE INVENTION It is an object of this invention to provide afusing device capable of being set without manual manipulation wherebylarge numbers of such fuse devices may be set at a single time withoutindividual attention to each of the fuse devices.

It is a further object of this invention to provide a pressureactuatable switch responsive to change in exswitch position, and,contacts disposed at the firstand second positions for 'electricallyengaging the aforementioned arm. In an illustrative embodiment of thisinvention, the translating mechanism includes an actuating cam coupledto the envelope portion and displaceable therewith, a ratchet cam biasedby suitable spring means to engage the actuating cam, and a splinehousing for receiving keys of the respective actuating and ratchet cams.Each of the actuating and ratchet cams has an engaging cam surface; therelative position of a key of one of the cams is displaced from theposition of the other key with respect to its cam surface so that upondisplacement of the actuating cam in response to an increase of externalpressure to a first.

level,'the cam surface of the actuating cam engages the cam surface ofthe ratchet cam to move the ratchet cam and the arm carried thereby fromits first to the second switch position. At least three splines areformed within the spline housing to permit the key of the ratchet cam'and therefore the ratchet cam to be set at the first and second switchpositions. 7

A further aspect of this invention resides in a reset mechanism wherebyuponincreasing the external pressure to a second level higher than thefirst level, the keys of the actuating andratchet cams are both free ofthe splines to permit movement, i.e., rotation, of the ratchet cam to aninitial or reset position under the influence of a suitable motivatingmeans, such as a clock spring.

A further feature of this invention resides in the employment of adamping iluidwithin the envelope to eliminate the possible adverseeffects of vibration or other rapid movement that would otherwise tendto change inadvertently the switch position of the arm.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantagesof the present invention will become more apparent by referring to thefollowing detailed description and accompanying drawings, in which:

. FIG. 1 is a cross-section view of the pressure actuatable switchingapparatus in accordance with the teachings of this invention;

FIG. 2 is an assembly drawing in perspective of the componentscomprising the pressure actuatable switching apparatus of FIG. 1;

FIG. 3 is a perspective, cut-away view of the spline housingincorporated into the pressure actuatable switching-apparatus as shownin FIGS. 1 and 2;

FIGS. 4A to 46 are linear representations of the relative position ofthe teeth of the actuating and ratchet cams with respect to the splinesof the spline housing as the external pressure is increased to a firstrelatively low pressure level and then decreased therefrom; FIGS. 5A to5D are linear views of the relationship between the teeth of the ratchetand actuating cams with respect to the splines of the spline housing asthe external pressure is increased to a second, higher level andthendecreased therefrom;

FIG. 6 is a top-plan view of the ratchet cam and its arm with respect toa plurality of switch positions; and

FIG. 7 is a schematic representation of a system for establishing firstand second levels of pressure within a chamber for receiving a pluralityof switch actuating .apparatus, as shown in FIGS. land DESCRIPTION OFTHE. PREFERRED EMBODIMENTS ,With regard to the drawings and inparticular to FIGS. 1 and 2, there is shown a pressure actuatableswitching apparatus in accordance with. theteachings of this invention,comprising a hermetically sealed envelope 10 for receiving a switchingmechanism therein. The envelope it) includes an outer housing 16, afirst end plate 18, and an upper or external bellows 14. The first endplate 18 issecured to one end of the outer housing 16, while th4e otherend of the outer housing 16 is partially enclosed by a second end member20, providing a mounting board for receiving a plurality of electricalcircuit elements identified by the numeral 38. As better shown in FIG.2, the circuit components 38 have associated therewith sets ofelectrical contacts 72, 74, 76 and 78 disposed at discrete, switchpositions. As shown in FIG. 1, the upper bellows 14 is hermeticallysealed to the outerhousing l6 and receives therein a translatingmechanism coupled to the external bellows- 14 and displaceable therewithunder a change of external pressure to permit an arm 42 carried by aratchet cam 40 to be displaced and in particular to be rotated tion, asshown in FIGS. 1 and: 2. v

' As more clearly shown in FIG. 3, the inner surface of from oneswitching position to the next as defined by the positions of the setsof contacts 72, 74, 76 and 78.

In the illustrative embodiment shown in FIG. 1, a plurality of terminals71 are disposed about the periphery of the outer housing 16 whereby anelectrical connection" may be made from the exterior of the envelope toa point within the envelope 10. A plurality of electrical connectors 73serve to interconnect the terminals 7 1 and corresponding circuitcomponents 38. Thus, the switch apparatus as described herein issuitable in a ing of the ratchet cam 40 and at the other end within 'aring 34 disposed upon a partition 22. The partition 22 I withinthepartition 22 to provide a limited communicadisposed against themember 20. An actuating cam 50 a and a ratchet cam 40 are disposed,within the spline housing 60 in a manner to be more fully described. Theactuating cam 50 includes a'post 52 mounted axially upon the top thereofto be coupled with the upper bellows 14; in particular, the post 52 isdisposed within an opening of a shaft bearing 68 secured as by gluing toa portion of the envelope and in particular .to the upper bellows 14.Upon a change, and in particular an increase, in the pressure of theexternal atmosphere with respect to the pressure within the envelope 10,the upper bellows 14 is compressed to move the 'shaftbearing 68 and theactuating cam-50 in a downward posithe annularly-shaped spline housing60 includes a plurality of fingers 64 for forming therebetween a seriesof splines'66 corresponding to the switch positions of the' sets of'conta'cts 72, 74, 76. and 78. As ismore clearly shown in FIG. 2,-theactuating cam 50includes a series of keys 54a, 54b, 54c,-etc.disposedabout the'periphery the keys 54a, 54b,'54c, etc., respectively.The ratchet cam 40 includes'a plurality of keys 44a, 44b, 44c, etc.

. disposed about the periphery thereof,,-andia plurality of teeth 46a,46b, 46c, etc. corresponding to the' teeth tion between the first andsecond chambers 26 and 24. A damping fluid is disposed within the outerhousing 10 to dampen the movement of the actuating and ratchet cams 50and 40 to prevent inadvertent motion such as vibration or shock frommoving the arm 42.- In order for the external bellows 14 to becompressed, the damping fluid must be forced through the restrictiveopening 28 from the first chamber 26 to the second chamber 24 whereby aninternal bellows 30 is in turn compressed. The internal bellows 30 isdisposed within the second chamber 24 and ismounted by a ring 19 on thefirst end member 18. The resulting damping action as the fluidcompresses the internal bellows 30 will slow the movements of theexternal bellows l4 and the coupled actuating and ratchet earns 50 and40; as a result, the switching mechanism will not be set inadvertentlyto the next switch position if subjected to'vibrationand shock impulses,asmay be occasioned in the aerial de livery of weapons. Theselecteddamping fluid should have 'a relatively high viscositystabilityover wide temperature ranges, be chemically inert, and presenta relatively high impedance, between the sets of terminals 72, 73, 74and 76 in orderto avoid electrically shorting these terminals-In oneillustrative.embodimentof 1 this invention, the damping fluidwas asilicone fluid.

57a, 57b, 570, etc. of-the actuating cam and config- 1 ured tomes'htherew'ith. Significantly, the keys 44 have a width of only onehalf or that of the teeth '46 and are disposedas shown in FIG. 2 fromthe tips of the teeth ;46. Ina manner to be explained, the displacementof the keys 44 with respectto the teeth 46 permits the roration ofthefratchetucarn"40 with respect to the spline housing to. facilitatethe successive movement of the arm '42,to the various switchpositionsfAs shown in FIG 2, the arm 42 carries a circuit completingmeans illustratively taking'the forin'of an'electrically conductivestrip 41 of "a metal Ora resistive film of selected imped'ance. Upondisposing the arm 42 at one of the switch positions, thejstrip'41- ispermittedto engage It is contemplated that thesecond cavity 24 'and theinner bellows 30 may be eliminated and'that the outer housing bepartially filled with the damping fluid; the resulting pocket of air, orbubble, will serve then as an internal compression chamber to dampen theeffect of external vibrations or shock upon the movement ofthe actuatingand ratchet cams. v

With regard to. FIG. 6,there is-shown the switching positions as definedby the sets of contacts 72, 74 and 78. In a manner to be explainedbelow,theratchet cam 40 is successively moved from thefirst to the second, toswitch successively positions as the external pressure is raised toafirst level with respectto thepressure within the envelope 10 and thenlowered, As; the ratchet cam .40 is 'rotated, the arm, and strip 41 aredisposed successively from one position, to the; next to electricallyengage the corresponding sets of contacts.

Themanner of interaction between' ratchet cam 40 and the actuating camsfltolrotatethe ratchet cam 40'trom one switch position tqthe nextwillnow be explained with regard .to', FIGS- 4A1Q14G, which showialinear representation of thecircumference of the actu ating andratchetcams and the spline housing. In FIG. 4A, the external pressurehas not yet been increased significantly with respect to theinternalpressure within r the envelope and the ratchet cam 40is'in oneof theset electrically the corresponding terminal set to complete acircuitjtherebetween.

' As shown.in-FIG..1, the ratchet cam 40 is' disposed' positions asdefined bythe splin'es 66 formed between splines66 as defined by. thefingers .64 of the. spline housing 60. Because the keys-54am confined bythe fingers 64, the. teeth of the" a u i dim-Chet cams.

are not permitted to mesh and between the cams.

With respect to FIG. 4B, the external pressure is increasing whereby theexternal bellows l4 forces the actuating cam 50 downward so that the camor teeth surfaces 55 of the actuatingcam 50 engage the teeth or camsurfaces 45 of the ratchet cam 40 to direct the ratchet cam 40 downward.As shown in FIG. 4B, ratchet cam 40 is restrained against rotationalmovement as its keys 44 are held by the fingers 64 of the an open spaceOCCUI'S spline housing 60. As the pressure continues to inratchet cam 40to the left, as shown in FIG. 4C, and

clockwise, as shown in FIG. 2. As the external'pressure decreases fromthe first level, the force exerted upon the actuating cam 50 is reducedand the spring 32 forces the ratchet cam 40 upward, as shown in FIGS. 4Cand 4D. As seen in FIG. 4D, the cam or teeth surfaces 45 of the ratchetcam 40 engage cam surfaces'65 of the fingers 64 of the spline housing60. As the external pressure continues to be reduced, .the axialforceexerted by the spring 32 moves the ratchet cam 40 upward whereby theinteraction of the cam surfaces 45 upon the cam surfaces 65 impartsafurther movement to the ratchet cam 40 in a direction to the left asseen 'in FIGS. 4E and 4F. In FIG. 4F, the key 44a'has been the keys 44and 54 to their topmost positions within the splines 66 of the splinehousing 60 ,(see FIG. 3). The translation imparted to the ratchet cam 40may be more fully realized by comparing FIGS. 4A and 4G. In FIG. 4A,thekey 44a is disposed between the fingers 64a and 64b. By increasingthe pressure to the first level and then releasing the pressure to anormal level, the ratchet cam 40 is displaced to the left as seen inFIGS. 4 to the next position as seen in FIG. 4G wherein the key 44a isdisposed between the fingers 64b and 64c of the spline housing 60. Duetothe axial force exerted upon the ratchet cam 40 by the spring 32, theswitching mechanism is now set in this switch position until the nextincrease and decrease in pressure.

, It is apparent that the above-described pressure increase and releasecycle may be performed any number of times to permit the ratchet cam 40and its arm 42 to be rotated in a counterclockwise direction as seen inFIG. 2 to each of at least four switch positions. Though only fourswitch positions are illustrated in FIG. 2, it is realized that anynumber of switch set positions may be provided in accordance with theteachings of this invention. 1

Further, the pressure actuatablev switching apparatus of this inventionmay include a reset feature whereby the arm 42 of the ratchet cam 40 isreset from any of the plurality of switch set positions as defined bythe contacts 74, 76 and 78 to its first or initial set position asdefined by the set of contacts 72. As seen in FIGS. 1 and 2, a clockspring 36 is interconnected between the ratchet cam 40 and the peripheryof the outer housing 16 to exert a counterclockwise force as seen inFIG. 2 upon the ratchet cam 40. Normally, the ratchet cam 40 is held ina set position because its keys 44 are confined within one of thesplines 66 of the spline housing 60 to prevent the rotation of theratchet cam 40 under the influence of the clock spring 36. When theratchet cam 40 and the actuating cam 50 are completely released from thesplines 66 of the spline housing 60 in a manner to be explained, theratchet cam 40 is rotated by the clock spring 36 in a counterclockwisedirection until the arm 42 is stopped by a post 21 mounted on the secondend member 20 whereby the ratchet cam 40 and its arm 42 are disposed atthe initial or first switch set position.

The operation of the reset mechanism as described above will'beexplained in greater detail with regard to FIGS. 5A to 5D, which show alinear representation of the relationship of the keys of the ratchet andactuating cams with respect to each other and to the fingers of thespline housing. In FIG. 5A, the ratchet cam 50 is shown disposed inafourth position whereby its key 44a is disposed between the fingers 64aand 1 64b of the spline housing 60. The external pressure outside theenvelope 10 is normal and the spring 32 retains the keys 44 and 54 fullywithin the splines 66 of the spline housing 60. In order to reset theratchet cam 40 to its first or initial position, the external pressureis increased with respect to the pressure within the envelope 10 to asecond level higher than the aforementioned first level, whereby theexternal bellows 14 disposes the actuating cam 50 and the ratchet cam 40to a position as shown in FIG. 5B and the axial force of the spring 32has beensufficiently overcome to free both the keys 54 of the actuatingcam 50 and the 'keys.44 of the ratchet cam 40 from the fingers 64 of thespline housing 60. At this point, the clock spring 36 is permitted torotate the ratchet cam in a counterclockwise direction as seen in FIG. 2(or to the right, as seen in FIGS. 5A to 5D) until the arm 42 oftheratchet cam 40 abuts the stop post 21, whereby the ratchet cam 50 isdisposed at its reset position, as shown in FIG. 5C. Then, the externalpressure is released toa normal level, whereby the spring 32 directs theratchet cam 40 in an upward direction as seen in FIG. 5D,,and the keys44 are fully disposed within the splines 66. When the pressure has beencompletely released, the ratchet cam 40 is disposed at its initialposition.

If the reset mechanism as described above is omitted, the pressureactuatable switch apparatus of this invention may be continuously set inone direction until the ratchet cam 40 has been redisposed in itsinitial position. In this embodiment, the mechanism may be considerablysimplified and the manufacturing tolerances are increased. However, if areset mechanism including the clock spring 36 is incorporated,continuous rotation of the ratchet cam 40 is not possible. Thus, theinclusion of the reset mechanism requires that reset be performed oncethe switching apparatus has been. set through all of its settings.

Though not shown in the drawings, a status indicator may be providedwhereby suitable indicia may be disposed upon the arm 42 of the ratchetcam 40 and a portion of the envelope 10 is made transparent to permit anoperator to observe the position of the arm 42 and thereby to identifythe set switch position to which the switching mechanism has been set.

With regard to FIG. 7, there is shown an illustrative system forselectively controlling the external pressure applied to a great numberof switch apparatus as described above. ln particular, a source 80 ofrelatively I high pressure is applied by a manual shut-off valve 82 toeach of a set" pressure regulator 88 and a reset pressure regulator 86.The pressure of the source 80 is. measured by a suitable gauge 84. Theset pressure regulater 88 provides at its output a pressurecorresponding to the first level whereby the switch apparatus disposedwithin a chamber 94, are set from one position, to the next, whereas thereset pressure regulator 86 provides an output of a greater pressurecorresponding to the second level whereby the switching apparatus arereset to their initial positions. lllustratively, the pressure of thefirst level derived from'the set pressure regulator 88 may be in theorder of 30 PSI, and the pressure derived from the reset pressureregulator 86 of the second level may be in the order of 60 PSI. Inorderto reset the. switch apparatus which are disposed within thepressure chamber 94, the operator manipulates the set control valve 92to initiate an increase in pressure within the pressure chamber 94. Apressure sensor and time delay 98 coupled with the chamber 94 measuresthe pressure build-up within the chamber 94 and latches the set controlvalve 92 until the firstlevel is reached.

After a delay determined by the pressure sensor and time delay 98corresponding to the damp'edresponse of the pressure actuatable switchapparatus, the vent valve 100 is opened underthe control of the pressuresensor and time delay 98 to permit the pressure within the chamber 94 tobe returned to a normal level. In this manner, each of the switchapparatus disposed with the chamber 94 is 'set or moved to its nextswitch set position.jln a similar manner, the operator may manipulatethe resetcontrol valve 90 to introduce a pressure of the second 'levelinto the chamber 94. A pressure sensor and time delay 96 measures thepressure within the chamber 94 and latches the reset control valves 90until a pressure of the second level is achieved. After a delaydependent upon the damping characteristics of the switch apparatus, thepressure sensor' and time delay 96 releases the pressure built up withinthe chamber 94 to be vented through the vent valve 100. Thus, theexternal pressure'applied to -a great number of switch apparatus may beraised to the second level,

whereby each of the switch apparatus is reset to its inia. an envelopefor receiving switch means therein and having a diaphragm portiondisplaceable in re- 1 sponse to changes of external pressure withrespect to that established withinsaid envelope;

8 b. first and second contacts disposed within said envelope, at firstand second switch positions, respectively; g I c. said .switch meanscomprising a switch member movable to each of said first andsecondswitch positions; d. translating means for securely setting saidswitch member at each of said first and second positions and responsiveto the displacement of said diaphragm portion to release said switchmember from said first position and move said switch member tosaidsecond position, said translating means effecting the movement ofsaid switch member from said first position to said second position inresponse to the increase of the external pressure from a normal pressurelevel to a first pressure level and the subsequent decrease of theexternal pressure from said first level to said normal pressure level;and e. reset means responsive to the increase of external pressure fromsaid normal level to a second level greater than said first level fordisposing said switch member to said first position. v t

2. Pressure actuatable switch apparatus comprising:

a. an envelope for receiving switch means therein and having a diaphragmportion displaceable in re-- sponse to changes of external pressure withrespect to that established within said envelope;

b. first and-second contacts disposed within said envelope at first andsecond switch positions, respectively;

c. said switch means comprising a switch member movable to each of saidfirst and second switch positions; v

d. translating means for securely setting said switch member at each ofsaid first and second positions and responsive to the displacementofsaid envelope portion to release said switch member from said firstswitch position andmove said switch member to said second switchposition, said translating means comprising an actuating cam coupled tosaid displaceable diaphragm portion for movement therewith, saidactuating cam having a first cam surface, and a ratchet cam forsupporting said switch member for movement therewith and having a secondcam surface for-engagement with said first cam surface whereby upondisplacement of said actuating camin response to a change of externalpressure, said ratchet cam ismoved so that said switch member isdisposed from said first to said second switch position,- a splinehousing .fixably mounted within said envelope for receiving saidactuating cam and said ratchet cam, said spline housing defining atleast first and second splines corre sponding .to said first andsecond-switch positions, said actuating and ratchet camseach having atleast one key disposable within'one of said splines,

and biasingmeans for urging'said ratchet cam to engage said actuatingca'm against displacement of said diaphragm portion; and

e. reset means comprisingspring means for urging said ratchet cam tosaid first position, stop means disposed within said envelope to engageat said first switch position said switch member as it isurged'by saidspring means, said actuating cam and, said ratchet cam being releasedfrom'said spline housing in response to an incr'ease'in pressureto alevel dependent upon the force exerted by sa'idspring means upon-saidratchet cam to permit movement by said spring means of said ratchet camandsaid switch member to said first switch position.

1. Pressure actuatable switch apparatus comprising: a. an envelope forreceiving switch means therein and having a diaphragm portiondisplaceable in response to changes of external pressure with respect tothat established within said envelope; b. first and second contactsdisposed within said envelope, at first and second switch positions,respectively; c. said switch means comprising a switch member movable toeach of said first and second switch positions; d. translating means forsecurely setting said switch member at each of said first and secondpositions and responsive to the displacement of said diaphragm portionto release said switch member from said first position and move saidswitch member to said second position, said translating means effectingthe movement of said switch member from said first position to saidsecond position in response to the increase of the external pressurefrom a normal pressure level to a first pressure level and thesubsequent decrease of the external pressure from said first level tosaid normal pressure level; and e. reset means responsive to theincrease of external pressure from said normal level to a second levelgreater than said first level for disposing said switch member to saidfirst position.
 2. Pressure actuatable switch apparatus comprising: a.an envelope for receiving switch means therein and having a diaphragmportion displaceable in response to changes of external pressure withrEspect to that established within said envelope; b. first and secondcontacts disposed within said envelope at first and second switchpositions, respectively; c. said switch means comprising a switch membermovable to each of said first and second switch positions; d.translating means for securely setting said switch member at each ofsaid first and second positions and responsive to the displacement ofsaid envelope portion to release said switch member from said firstswitch position and move said switch member to said second switchposition, said translating means comprising an actuating cam coupled tosaid displaceable diaphragm portion for movement therewith, saidactuating cam having a first cam surface, and a ratchet cam forsupporting said switch member for movement therewith and having a secondcam surface for engagement with said first cam surface whereby upondisplacement of said actuating cam in response to a change of externalpressure, said ratchet cam is moved so that said switch member isdisposed from said first to said second switch position, a splinehousing fixably mounted within said envelope for receiving saidactuating cam and said ratchet cam, said spline housing defining atleast first and second splines corresponding to said first and secondswitch positions, said actuating and ratchet cams each having at leastone key disposable within one of said splines, and biasing means forurging said ratchet cam to engage said actuating cam againstdisplacement of said diaphragm portion; and e. reset means comprisingspring means for urging said ratchet cam to said first position, stopmeans disposed within said envelope to engage at said first switchposition said switch member as it is urged by said spring means, saidactuating cam and said ratchet cam being released from said splinehousing in response to an increase in pressure to a level dependent uponthe force exerted by said spring means upon said ratchet cam to permitmovement by said spring means of said ratchet cam and said switch memberto said first switch position.